Fix WidgetTest.ChildStackedRelativeToParent on 10.13

Since 10.13, a trip to the runloop seems to be been necessary to ensure
[NSApp orderedWindows] updates inside WidgetTest::IsWindowStackedAbove().
Since tests using this are only concerned with relative ordering of
windows in the same process, flushing the run queue here shouldn't cause
flakiness.

Bug: 749905
Change-Id: I2fdd42994f759613d91458bf4c2d15cb20cf3c65
Reviewed-on: https://chromium-review.googlesource.com/590332
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490911}
1 file changed